Handover (HO) indicates that a mobile station (MS) moves from an air interface of one base station to an air interface of another base station. Hereinafter, a handover procedure of a general IEEE 802.16e system will be described.
In an IEEE 802.16e network, a serving base station (SBS) may broadcast neighboring base station information through a neighboring advertisement (MOB_NBR-ADV) message in order to inform a mobile station (MS) of information (topology) about a basic network configuration.
The MOB_NBR-ADV message includes system information of a serving base station and neighboring base stations, for example, preamble indexes, frequencies, HO optimization degrees, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
DCD/UCD information includes information necessary to enable an MS to perform information exchange in downlink and uplink. For example, DCD/UCD information includes HO trigger information, information about a medium access control (MAC) version of a base station, media independent handover capability information, etc.
A general MOB_NBR-ADV message includes only information about neighboring base stations of an IEEE 802.16e type. Information about base stations of the type other than an IEEE 802.16e type may be broadcast to MSs through a service identity information advertisement (SII-ADV) message. Accordingly, an MS may request an SBS to transmit the SII-ADV message so as to acquire information about base stations of a heterogeneous network.
A procedure for performing handover by an MS, which acquires information about neighboring base stations using the above-described method, in an IEEE 802.16e network will be described in greater detail with reference to FIG. 1.
FIG. 1 is a diagram showing an example of a handover procedure which may be performed in a general IEEE 802.16e system.
Referring to FIG. 1, first, a mobile station (MS) may access a serving base station (SBS) so as to perform data exchange (S101).
The SBS may periodically broadcast information about neighboring base stations (BSs) located within a coverage area thereof to the MS through an MOB_NBR-ADV message (S102).
The MS may begin to scan candidate HO BSs using an HO trigger condition while communicating with the SBS. The MS may transmit a handover request (MOB_MSHO-REQ) message so as to request the SBS to perform a handover procedure if a handover condition exceeds, for example, a predetermined hysteresis margin value (S103).
The SBS may inform candidate HO BSs included in the MOB_MSHO-REQ message of the handover request from the MS through an HO-REQ message (S104).
The candidate HO BSs may take precautions for the MS which requests handover and transmit information associated with handover to the SBS through an HO-RSP message (S105).
The SBS may transmit information associated with handover acquired from the candidate HO BSs through the HO-RSP message to the MS through a handover response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may include information for performing handover, such as an action time for performing handover, a handover identifier HO-ID and a dedicated HO CDMA ranging code (S106).
The MS may determine one target BS among the candidate HO BSs based on information included in the MOB_BSHO-RSP message received from the SBS. Then, the MS may transmit a CDMA code to the determined target BS so as to attempt to perform ranging (S107).
The target BS which receives the CDMA code may transmit information as to whether or not ranging succeeds and physical correction values to the MS through a ranging response (RNG-RSP) message (S108).
The MS may transmit a ranging request (RNG-REQ) message for authentication to the target BS (S109).
The target BS which receives the ranging request message of the MS may provide system information available to the BS, such as a connection identifier (CID), to the MS through a ranging response message (S110).
If the target BS successfully completes the authentication of the MS and sends all update information, the target BS may inform the SBS of the MS of information indicating whether or not handover succeeds through a handover completion (HO-CMPT) message (S111).
Thereafter, the MS may exchange information with the target BS which performs handover (S112).
The above-described handover process is performed between the MS and the BS which follow the IEEE 802.16e standard (WirelessMAN-OFDMA R1 Reference System). In the handover procedure defined in the IEEE 802.16m (WirelessMAN-Advanced Air Interface) system, kinds of a medium access control (MAC) management message and the parameters included therein may be partially different. For example, a ranging request/response (RNG-REQ/RSP) message is replaced with an advanced ranging request/response (AAI-RNG-REQ/RSP) message and a handover response (BSHO-RSP) message is replaced with a handover command (AAI-HO-CMD) message.
Hereinafter, in the present specification, for convenience, a system to which a general technology including the IEEE 802.16e standard is applied is referred to as a “legacy system” or a “R1 system”. An MS to which the legacy technology is applied is referred to as a “legacy MS” or a “R1 MS”. An operation mode of an MS or BS to which the general technology is applied is referred to as a “legacy mode”.
An MS to which an advanced technology including the IEEE 802.16m standard (WirelessMAN-OFDMA Advanced Air Interface) is applied is referred to as an “advanced MS”. A BS to which the advanced technology is applied is referred to as an “advanced BS (ABS)”. An operation mode of an MS or BS to which the advanced technology is applied is referred to as an “advanced mode”.
It is assumed that an AMS accesses a legacy BS so as to receive a service from the legacy BS, and an ABS (WirelessMAN-OFDMA R1 Reference System/WirelessMAN-OFDMA Advanced co-existing system) supporting both an AMS and a legacy MS is located adjacent to the legacy BS. Hereinafter, an ABS supporting both the AMS and the legacy MS, that is, the legacy mode and the advanced mode, is referred to as a “mix-mode ABS” and an operation mode of such a BS is referred to as a “mixed mode”.
The legacy BS has only a legacy zone (LZone) having a physical channel frame structure applied to a legacy system. It is assumed that an ABS has only an AMS support zone (MZone: 16M zone) having a physical channel frame structure applied to an advanced system if only an AMS is supported (WirelessMAN-OFDMA advanced system only). An ABS (WirelessMAN-OFDMA R1 Reference System/WirelessMAN-OFDMA Advanced co-existing System: legacy supportive) which supports both an AMS and a legacy MS has both a legacy zone and an AMS support zone, which are divided in time units, for example, is divided using time division duplex (TDD) in frame units or subframe units, in uplink and downlink.
It is assumed that the AMS may receive services from both the ABS and the legacy BS. That is, it is assumed that the AMS may receive a service through any one of the AMS support zone and the legacy zone and may perform both a handover procedure defined in the legacy system and a handover procedure defined in the advanced system.
In order to perform handover from a serving legacy BS to an ABS supporting both an AMS and a legacy MS, an AMS may first enter the legacy zone of the ABS and then continuously receive a service in the legacy zone or perform zone switch to the AMS support zone. Alternatively, the AMS may perform handover using a method of performing zone switch to an AMS support zone without entering the legacy zone of the ABS.
Zone switch refers to a procedure of enabling an AMS which has operated in the LZone to operate in the MZone, as the LZone and the MZone which is divided using TDD is present in one carrier. That is, an MS switches a MAC operation defined in the IEEE 802.16e system to a MAC operation defined in the IEEE 802.16m system. In contrast, the AMS may move from the MZone to the LZone.
In order to enable the MS to perform zone switch in the same BS, the MS and the target BS should be aware of some information in advance. For example, the target BS should be aware of whether the MS supports the IEEE 802.16m system in order to schedule a handover or zone switch procedure. The MS should be aware of whether the target BS supports the IEEE 802.16m system and information about a zone to which the MS will move. More specifically, if the MS performs zone switch from the LZone to the MZone, the MS should receive system information (that is, a superframe header (SFH)) of the MZone and be aware of the start location of the MZone in a frame structure.
There are two methods of performing handover defined in the IEEE 802.16e system to the LZone of a mix-mode BS and then performing zone switch to the MZone, that is, a zone switch mode 0 and a zone switch mode 1. In the zone switch mode 0, the MS breaks connection with the LZone at a specific time (e.g., a zone switch action time), performs synchronization with the MZone and receives system information (SFH). The MS which successfully completes the above procedure begins to enter a network of the MZone. At this time, latency occurs due to synchronization and SFH reception time.
In contrast, in the zone switch mode 1, the MS may maintain the operation which has been performed in the LZone and, at the same time, perform synchronization with and network re-entry to the MZone. Accordingly, latency does not occur during zone switch.
The zone switch mode 1 is preferable in terms of latency. However, in terms of implementation of the MS, the MS should perform communication with two zones and thus a large burden is placed upon the MS. In contrast, the zone switch mode 0 is advantageous in terms of implementation of the MS, but significant latency occurs and significantly influences Quality of Service (QoS). This is because the MS should receive the system information of the MZone in order to enter the network of the MZone as described above. Since the MS which will perform zone switch in the zone switch mode 0 is not aware of a start point of a superframe and a frame configuration of the MZone, the MS should perform zone switch while scanning all sections of the MZone from the specific time (e.g., zone switch action time).